Production method for porous metal body

ABSTRACT

A process for producing a metal body comprises (1) a step of maintaining under reduced pressure a metal material within a temperature range of room temperature to a temperature lower than a melting point of the metal in a sealed vessel to thereby degas the metal material, (2) a step of melting the metal material under pressurization by introducing a gas into the sealed container to thereby dissolve the gas into the molten metal, and (3) a step of cooling and solidifying the molten metal while controlling a gas pressure and a molten metal temperature in the sealed vessel to thereby form a porous metal body.

This application is the U.S. National Phase under 35 U.S.C. §371 ofInternational Application PCT/JP00/04567, filed Jul. 10, 2000, whichclaims priority to Japanese patent application Ser. No. 11/195,260,filed Jul. 9, 1999. The International Application was published underPCT Article 21(2) in a language other than English.

TECHNICAL FIELD

This invention relates to a process for producing a porous metal body.

There are known porous metal bodies and methods for producing them. Forinstance, the specification of U.S. Pat. No. 5,181,549 discloses aprocess for producing a porous metal body by dissolving hydrogen orhydrogen-containing gas in a molten raw metal material underpressurization, and then cooling and solidifying the molten metal underthe condition of controlling the temperature and pressure.

However, this method has some serious practical problems. For example,(1) an ultra-pure metal must be used as the raw material in order toobtain a porous metal body having excellent characteristics, (2) oxygen,nitrogen, hydrogen or other impurities, if contained in the raw metalmaterial, remain in the porous metal body and impair the characteristicsof the resulting porous metal body, limiting the field of use of theporous metal body, and (3) since hydrogen or hydrogen-containing gas isused as a gas to be dissolved in molten metal, the metal species to beused are limited to those giving a porous metal body which is notsubject to the impairment of characteristics due to hydrogen absorption.

DISCLOSURE OF THE INVENTION

The inventor conducted researches in light of the above-mentionedproblems encountered with the prior art porous metal body producingtechnology, and as a result discovered that a high-quality porous metalbody can ultimately be obtained by lowering the amount of impuritiescontained in the metal to or below a specific value before and duringthe melting of the raw metal material.

More specifically, the present invention provides the followingprocesses for producing a porous metal body.

1) A process for producing a porous metal body comprises the steps of:

(1) maintaining under reduced pressure a raw metal material within atemperature range from room temperature to a temperature lower than themelting point of the metal in a sealed vessel to thereby degas the rawmetal material;

(2) melting the raw metal material under pressurization by introducing agas into the sealed vessel to thereby dissolve the gas in the moltenmetal; and

(3) cooling and solidifying the molten metal in a mold while controllingthe gas pressure and the temperature of the molten metal inside thesealed vessel to thereby obtain the porous metal body.

2) In the process for producing a porous metal body according to item 1)above, the metal is selected from the group consisting of iron, copper,nickel, cobalt, magnesium, titanium, chromium, tungsten, manganese,molybdenum, beryllium, and alloys comprising one or more of thesemetals.

3) In the process for producing a porous metal body according to item 1)above, the reduced pressure in step (1) is 10⁻¹ Torr or lower.

4) In the process for producing a porous metal body according to item 3)above, the reduced pressure in step (1) is between 10⁻¹ and 10⁻⁶ Torr.

5) In the process for producing a porous metal body according to item 1)above, the metal material in step (1) is maintained at a temperaturewhich is 50 to 200° C. lower than the melting point of the metal.

6) In the process for producing a porous metal body according to item 1)above, the gas used in steps (2) and (3) is at least one member selectedfrom the group consisting of hydrogen, nitrogen, argon and helium.

7) In the process for producing a porous metal body according to item 1)above, the pressure applied in step (2) is between 0.1 and 10 MPa.

8) In the process for producing a porous metal body according to item 7)above, the pressure applied in step (2) is between 0.2 and 2.5 MPa.

9) In the process for producing a porous metal body according to item 1)above, the molten metal is poured in step (3) from the sealed vesselinto the mold equipped with a cooling apparatus.

10) In the process for producing a porous metal body according toitem 1) above, the cooling and solidification of the molten metal instep (3) is performed by a continuous casting method.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow diagram illustrating the general outline of producingsteps of the porous metal body according to the present invention.

FIG. 2 is a phase diagram showing phase change in an iron-nitrogensystem.

FIG. 3 is a conceptual diagram showing the gas-dissolvingcharacteristics of the solid and liquid phases in the cooling andsolidifying step of the molten metal in which gas has been dissolved.

FIG. 4 is a graph showing in detail the amount of nitrogen dissolved inpure iron (99.99%) above and below the melting point of the pure iron.

FIG. 5 is a graph showing the relationship between nitrogen/argonpartial pressure ratio and porosity in porous iron materials obtainedwhen pure iron (99.99%) is melted and cast under pressurization with anitrogen-argon mixed gas with different partial pressures.

FIG. 6 is a graph showing the relationship between nitrogen partialpressure and porosity in porous iron materials obtained when pure iron(99.99%) is melted and cast under pressurization with a nitrogen-argonmixed gas with different partial pressures under constant total pressureof 2.1 MPa.

FIG. 7 is a graph showing the relationship between nitrogen partialpressure and nitrogen content in porous iron materials obtained whenpure iron (99.99%) is melted and cast under pressurization with anitrogen-argon mixed gas with different partial pressures under constanttotal pressure of 2.1 MPa.

FIG. 8 is a cross section outlining the porous metal body producingapparatus used in the present invention.

FIG. 9 is a figure outlining a mold equipped with a cooling mechanism atthe bottom.

FIG. 10 is a figure outlining a cylindrical mold equipped with a coolingmechanism on its inner surface.

FIG. 11 is a cross section outlining the apparatus for producing aporous metal body by continuous casting method used in the presentinvention.

FIG. 12 is a figure outlining an apparatus for producing a rod- orplate-shaped porous metal material by continuous casting method.

FIG. 13 is a cross section outlining an apparatus for producing a rod-or plate-shaped porous metal material by continuous casting method.

FIG. 14 (a) to FIG. 14 (h) are partially cut-away oblique views ofporous metal materials in various forms which can be manufactured by themethod of the present invention.

FIG. 15 is a graph showing the relationship between partial gas pressureratio and porosity for four different porous copper materials obtainedby melting at 1250° C. under pressurization of 0.8 MPa withhydrogen-argon mixed gas.

FIG. 16 (a) to FIG. 16 (d) are electronically processed images(corresponding to optical micrographs) showing the pore distributionstate of four different porous copper materials obtained by melting at1250° C. under pressurization of 0.8 MPa with hydrogen-argon mixed gas.

FIG. 17 is an electronically processed image (corresponding to a12.5-power optical micrograph) illustrating a vertical cross section ofa cylindrical porous copper material having a shape corresponding toFIG. 14 (c).

FIG. 18 is a graph showing the relationship between partial gas pressureratio and porosity of the porous iron materials obtained by melting at1650° C. under pressurization of 1.5 MPa or 2.0 MPa with nitrogen-heliummixed gas.

FIG. 19 (a) to FIG. 19 (d) are electronically processed images(corresponding to optical micrographs) illustrating the poredistribution state of four different porous ordinary steel materialsobtained by melting at 1650° C. under pressurization with four differentnitrogen-helium mixed gases with various partial gas pressure ratios.

FIG. 20 is an electronically processed image (corresponding to anoptical micrograph) illustrating the pore distribution state of a porousnickel material (porosity: 17%) obtained by melting at 1600° C. underpressurization of 0.8 MPa with nitrogen-helium mixed gas.

FIG. 21 is an electronically processed image (corresponding to anoptical micrograph) illustrating a cylindrical porous copper materialobtained by melting at 1250° C. under pressurization of 0.9 MPa withhydrogen-argon mixed gas.

FIG. 22 is an electronically processed image (corresponding to anoptical micrograph) showing a cross section of the pore shape in thethickness direction of the cylindrical porous copper material shown inFIG. 21.

FIG. 23 is an electronically processed image (corresponding to anoptical micrograph) showing the surface state of the cylindrical porouscopper material shown in FIG. 21.

FIG. 24 is an electronically processed image (corresponding to anoptical micrograph) showing a cylindrical porous copper materialobtained by melting at 1250° C. under pressurization of 0.5 MPa withhydrogen-argon mixed gas.

FIG. 25 an electronically processed cross-sectional image (correspondingto an optical micrograph) showing the pore shape in the thicknessdirection of the cylindrical porous copper material shown in FIG. 24.

FIG. 26 is an electronically processed image (corresponding to anoptical micrograph) showing the surface state of the cylindrical porouscopper material shown in FIG. 24.

FIG. 27 is an electronically processed image (corresponding to anoptical micrograph) showing a transverse cross section of a porouscopper cylinder (diameter approximately 100 mm) obtained by melting at1250° C. under pressurization of 0.8 MPa with hydrogen-argon mixed gas.

EMBODIMENTS OF THE INVENTION

In the present invention, as shown in FIG. 1, first the metal whichserves as the raw material for producing a porous body is placed in avessel with a sealed construction, and the raw metal material is keptunder reduced pressure within a temperature range from normaltemperature to a temperature less than the melting point of the metal tothereby degas the metal material (step (1)).

Next, the degassed metal material is heated under pressurization with agiven gas to thereby melt the metal material and dissolve the gas in themolten metal (step (2)).

Then, while controlling the pressure of the gas and the temperature ofthe molten metal in the sealed vessel according to the type ofpressurizing gas and raw metal material, the molten metal is cooled andsolidified to thereby form a desired porous metal body (step (3)).

Usable as the raw metal materials are iron, copper, nickel, cobalt,magnesium, aluminum, titanium, chromium, tungsten, manganese,molybdenum, beryllium, and an alloy comprising one or more of thesemetals.

The degassing may be performed by placing a raw metal material composedof a suitable combination of two or more kinds of simple substancemetals in the sealed vessel. Alternatively, as the raw metal material, aconcomitant use of at least one simple substance metal and at least onealloy, or a concomitant use of two or more alloys is possible. In thesecases, an alloy is formed in the melting step which will be discussedbelow and the porous alloy material is ultimately obtained.

How much the pressure is reduced in step (1) varies depending on thetype of raw material metal and on the impurity components (such asoxygen, nitrogen and hydrogen) which are contained in the raw materialmetal and should be removed, but the pressure is usually 10⁻¹ Torr orlower, preferably within the range of 10⁻¹ to 10⁻⁶ Torr. If the pressurereduction is insufficient, the remaining impurity components may impairthe corrosion resistance, chemical resistance, toughness and so forth ofthe porous metal body. On the other hand, excessively reduced pressureimproves the performance of the resulting porous metal body somewhat,but increases the costs of producing and operating the apparatus, henceundesirable.

The temperature at which the raw metal material is maintained in step(1) is between ordinary temperature and a temperature lower than themelting point of the raw metal material (when two or more differentmetals are used together, lower than the lowest melting point), andpreferably about 50 to 200° C. lower than the melting point. Theoperation is easier if the degassing is performed by placing the rawmetal material in the sealed vessel at normal temperature, and thengradually raising the temperature. To enhance the degassing effect, itis preferable to heat the raw metal material at a temperature which isas high as possible but still under the melting point of the raw metalmaterial, prior to the start of step (2). When the raw metal material ismaintained at a higher temperature in step (1), the time required formelting the metal to be discussed below is shorter.

The time period during which the metal is maintained in step (1) may besuitably determined depending on the type and amount of impuritiescontained in the metal, the extent of degassing required and the like.

The degassed raw metal material is then melted under pressurization instep (2). As the pressurizing gas, one or more of hydrogen, nitrogen,argon and helium is used.

If safety is of particular importance, it is preferable to use at leastone of nitrogen, argon and helium as the pressurizing gas. It is alsopreferable to use a nitrogen-argon mixture, a nitrogen-helium mixture ora nitrogen-argon-helium mixture in order to more accurately control theporosity and pore size within the porous metal body.

In this step (2), part of the gas is dissolved in the molten metal underpressurization. As shown in the metal-gas system phase diagram shown inFIG. 2, it is preferable that the amount of gas dissolved in the moltenmetal falls within a certain range including a formation amount at theeutectic point C₃ under the given pressurization conditions. The amountof gas dissolved in the molten metal is determined by taking intoaccount such factors as the type of metal, the type of gas, the gaspressure, and the desired pore structure of the porous metal body.

The pressurization condition in step (2) is determined according to thetype of metal and the pore shape and diameter, the porosity and so forthof the porous metal body ultimately obtained, but is usually about 0.1to 10 MPa, more preferably 0.2 to 2.5 MPa.

Any pressurizing gas may be selected from the group of gases givenabove, as long as it does not inhibit the characteristics of the porousmetal body eventually obtained. However, there are preferredcombinations of metal and gas. Examples of such preferred combinationsinclude iron-nitrogen/argon (“nitrogen/argon” means a mixed gas ofnitrogen and argon; the same applies hereinafter), iron-nitrogen/helium,an iron alloy (industrial-grade pure iron, ordinary steel, stainlesssteel, etc.)-nitrogen/argon, an iron alloy (ordinary steel, stainlesssteel, etc.)-nitrogen/helium, copper-argon, copper-hydrogen,copper-hydrogen/argon, and nickel-nitrogen/argon.

The molten metal in which gas has been dissolved is then sent to step(3), where it is cooled and solidified. As shown schematically in FIG.3, the amount of gas dissolved in the metal differs dramatically aboveand below the melting point. Specifically, a large quantity of gasdissolves in metal in a molten state, but as the temperature falls andthe metal begins to solidify, there is a sharp reduction in the amountof dissolved gas. Therefore, by solidifying the molten metal in acertain direction while suitably controlling the temperature of themolten metal and the gas atmosphere pressure, bubbles can be produced inthe solid phase portion near the solid phase/liquid phase interface dueto the separation of gas which has been dissolved to supersaturation inthe liquid phase portion. Since these gas bubbles grow as the metalsolidifies, numerous pores are formed in the solid phase portion. Inthis step (3), as will be discussed in further detail below, a porousmetal body in which the pore shape, pore diameter, porosity and so forthare controlled as desired is obtained by controlling the cooling rate orthe solidification rate of the molten metal and suitably adjusting thecomposition of the solidification gas atmosphere (the mixing ratio ofnitrogen gas/inert gas) and the gas pressure (increasing the pressure,maintaining the pressure, or reducing the pressure).

FIG. 4 is a graph illustrating in detail the change in the amount ofdissolved nitrogen (the left vertical axis shows concentration in theliquid phase, and the right vertical axis shows concentration in thesolid phase) in pure iron (99.99%) that has been maintained underpressurization of 2.3 MPa with a nitrogen/argon mixed gas.

As is clear from FIG. 4, the nitrogen solubility in liquid iron andsolid iron varies sharply and irregularly in the transition from themelt to solidification of pure iron. Even in solidified iron, successiveallotropic transformation occurs from a δ phase, through a γ phase, toan α phase and the amount of dissolved nitrogen varies as thetemperature drops. This difference in nitrogen solubility can beutilized to form pores in solid iron by means of the nitrogen gasseparated out in the γ phase. This phenomenon also occurs in the samemanner when nitrogen-inert gas mixture, hydrogen-nitrogen mixture,hydrogen-inert gas mixture, hydrogen-nitrogen-inert gas mixture or thelike is used instead of nitrogen as the pressurizing gas, so that asimilar porous iron material can be obtained. Furthermore, the similarphenomenon occurs when an iron alloy such as steel, copper or an alloythereof, nickel or an alloy thereof, or any of the various metals listedabove or an alloy thereof is used as the metal species, so that porousbodies of various metals can be produced by the same procedure.

Also, a certain correlation is generally seen between the gas atomconcentration in a metal-gas system and the state of pore formation(pore distribution, pore size, etc.) in the manufacture of a porousmetal body at a constant pressure. We will assume here that thegas-dissolved metal (metal-gas system) is cooled in a cylindrical moldfrom the circumferential surface direction, and that we are observing across section of the cylindrical metal body thus obtained. Here, if thecooling is carried out properly, substantially the same results will beobtained no matter where the cross section is located.

First, as shown in FIG. 2, if the gas atom concentration C₁ isconsiderably lower than the eutectic composition C₃, in the course ofcooling from a temperature T₁ to T_(E), a non-porous metal solid phaseportion is formed in a certain thickness from the inner surface of themold toward the center, and then in the course of cooling from thetemperature T_(E) to a lower temperature, a porous metal phase is formedin the middle region (see cross section C₁).

If the gas atom concentration C₂ is between the eutectic composition C₃and C₁, in the course of cooling from a temperature T₂ to T_(E), anon-porous metal solid phase portion is formed in a narrower width fromthe inner surface of the mold toward the center, and then in the courseof cooling from the temperature T_(E) to a lower temperature, a porousmetal phase is formed in a broader middle region (see cross section C₂).

If the metal-gas system has the eutectic composition C₃, the metalbegins to solidify at the temperature T_(E) and pores are formed at thesame time, so that non-porous metal solid phase portion is formed. Thepore size is relatively uniform (see cross section C₃).

If the gas atom concentration C₄ is higher than a eutectic compositionC₃, in the course of cooling from a temperature T₄ to T_(E), large poresare formed in the liquid phase, and the metal begins to solidify at thetemperature T_(E). Smaller pores are formed in the course of coolingfrom the temperature T_(E) to a lower temperature. Therefore, in thiscase a porous metal phase including pores of different sizes is formed,and no non-porous metal solid phase portion is formed (see cross sectionC₄).

FIG. 5 is a graph showing an example of the change in porosity in porouspure iron (99.99%) manufactured under pressurization with a mixed gas ofnitrogen and argon. As is clear from FIG. 5, when the argon gas pressureis constant, the porosity in the porous body increases as the nitrogengas pressure increases. Conversely, when the nitrogen gas pressure isconstant, the porosity in the porous metal body decreases as the argongas pressure increases. As indicated by the three broken lines, theporosity in the porous body tends to increase as the gas pressure of theentire mixed gas increases.

FIG. 6 is a graph showing an example of the change in porosity in porouspure iron (99.99%) manufactured under constant pressure pressurization(2.1 MPa) with nitrogen-argon mixed gas. As is clear from FIG. 6, underconstant pressure conditions, the porosity in the porous body increasesalong with the increase in the nitrogen partial pressure. If FIGS. 5 and6 are considered together, it is clear that nitrogen gas contributesgreatly to an increase in the porosity in the porous metal body. Similarresults were also obtained when nitrogen-helium mixed gas is usedinstead of nitrogen-argon mixed gas.

It is clear from the results shown in FIGS. 5 and 6 that the porosity ofa porous metal body can be controlled by adjusting the composition ofthe pressurization atmosphere gas.

FIG. 7 shows the nitrogen content in porous pure iron (99.99%)manufactured under constant pressure pressurization (2.1 MPa) with anitrogen-argon mixed gas. The nitrogen content steadily rises along withthe rise in nitrogen partial pressure, but saturates when the nitrogenpartial pressure is about 1 MPa. The obtained porous pure iron has ahigh apparent nitrogen content, but the majority of this nitrogen isconcentrated in an extremely thin surface-layer portion on the surfaceof the pores, and only a trace amount of Fe₄N is contained and dispersedin the a phase in the interior of the pure iron. That is to say, thehardness of the resulting porous body is markedly improved, as if theentire surface, including the pore surfaces, had been subjected tonitriding treatment. This distinctive aspect of the entire porous body,in which only a trace amount of Fe₄N is present in the interior eventhough a large quantity of nitrogen is contained in the porous body as awhole, is presumably attributable to the subtle changes in the amount ofdissolved nitrogen due to the transition from the liquid phase to thesolid phase (δ phase, γ phase and α phase).

The porous metal body obtained with the present invention also hasvarious other excellent characteristics (such as its strength,toughness, machinability, workability, weldability, vibrationattenuation, acoustic attenuation, high specific surface area, etc.).For example, the porous metal material according to the presentinvention has a specific strength (strength/weight) which is about 20 to30% higher than that of the raw metal material, and the Vickers hardnesswhich is about three times higher.

The iron-based porous metal body obtained by the present invention canalso be further hardened by hardening treatment to increase its Vickershardness to about twice that prior to the hardening.

FIG. 8 is a cross section showing an example of the apparatus used inthe present invention to manufacture a porous metal body.

The apparatus shown in FIG. 8 has a raw metal material heating andmelting section 1 and a molten metal cooling and solidifying section 2,which are the main constituents, disposed one above the other.

The raw metal material heating and melting section 1 comprises a metalmelting tank 4, an inductive heating coil 7, a stopper 8, a degassingpath 31, a gas introduction pipe 9, and a gas exhaust pipe 10. In step(1), the raw metal material is placed in the melting tank 4, and thenthe stopper 8 is placed in its closed position to seal off the meltingtank 4, and a vacuum pump (not shown) is then actuated to purge the gasinside the melting tank 4 through the degassing path 31 and to achievethe desired reduced pressure condition. Electric power is then suppliedto the inductive heating coil 7, and the raw metal material is heatedaccording to a given heating profile under reduced pressure. Thisheating treatment under reduced pressure greatly reduces the amount ofimpurity gas components, such as oxygen, nitrogen and so forth in theraw metal material. As a result, the gas content in the porous metalbody eventually obtained is also greatly reduced.

Then, a gas is introduced from the gas introduction pipe 9 into an upperspace 3-b of the melting tank 4 while the impurity gas componentsreleased from the raw metal material are purged through the gas exhaustpipe 10 to the outside of the melting tank.

In step (2), with the gas exhaust pipe 10 closed, a given gas isintroduced from the gas introduction pipe 9 into the upper space 3-b ofthe melting tank 4, and the metal is melted by supplying electric powerto the inductive heating coil 7 either while or after the inside of themelting tank 4 is pressurized to the specified pressure. Thepressurizing gas in step (2) and the purging gas in step (1) may havethe same or different compositions, but from the standpoints ofsimplifying the gas supply apparatus, facilitating gas-supply operationand so forth, it is preferable that the compositions are the same. Bymelting the metal under this pressurization conditions, a large quantityof gas is dissolved in the metal, as shown in FIG. 3 and FIG. 4.

Subsequently, the stopper 8 is lifted and the molten metal 3-a in whichthe gas has been dissolved is poured through a molten metal inlet 11into a mold 5 disposed at the bottom of the molten metal cooling andsolidifying section 2, forming a porous metal body. Before the moltenmetal is poured in, a given gas is introduced from a gas supply pipe 12into the molten metal cooling and solidifying section 2 so as tomaintain the interior thereof at the specified pressure. The gaspressure inside the molten metal cooling and solidifying section 2 canbe easily controlled by suitably opening or closing the gas supply pipe12 and a gas exhaust pipe 13. Meanwhile, the cooling rate of the moltenmetal inside the mold 5, which is equipped with a cooling mechanism 6,can be controlled by the amount of a cooling water that is supplied froma pipe 14 for introducing water or like coolant (since water is usuallyused, this will hereinafter be referred to as “water”) and dischargedfrom a cooling water discharge pipe 15.

Thus, by cooling the molten metal poured in the mold 5 from the bottomby means of the cooling mechanism 6 while controlling the gas pressureinside the melted metal cooling and solidifying section 2, numerousbubbles originating from the gas dissolved in the liquid phase portionare produced near the interface between the liquid phase on the top andthe solid phase on the bottom, and these bubbles create pores in thesolid phase. As a result, a porous metal material having the given poreshape, porosity and so forth is obtained.

FIG. 9 is a drawing schematically illustrating an example of the mold 5and its cooling mechanism 6 used in the apparatus shown in FIG. 8. Inthis embodiment, the cooling mechanism 6 itself serves as the bottom ofthe mold 5. In this case, cooling water is supplied from the bottom ofthe cooling mechanism 6 which is in contact with the molten metal 3-a,thereby rapidly cooling the molten metal. Although FIG. 6 shows thestate when vertical pores are being formed in the course of cooling themolten metal, a porous metal body 3 having pores extending verticallyfrom bottom to top can be eventually formed as the metal solidifies.

FIG. 10 is a simplified diagram showing another example of the mold 5and its cooling mechanism 6 used in the apparatus shown in FIG. 8. Inthis embodiment, the cooling mechanism 6 is disposed in the center ofthe mold 5, and the molten metal 3-a is poured into the cylindricalspace in between the two. Although FIG. 10 shows the state when lateralpores are being formed in the course of cooling the molten metal, aporous metal body 3 having pores extending laterally from the inside tothe outside of the cylinder can be eventually formed.

FIG. 11 schematically illustrates an example of a porous metal bodyproducing apparatus featuring continuous casting method.

The apparatus shown in FIG. 11 has a raw metal material heating andmelting section 1 and a molten metal holding section 22 disposed oneabove the other, and a continuous casting apparatus is linked in thelateral direction to the molten metal holding section 22. The degassingand melting of the raw metal material in the raw metal material heatingand melting section 1 are performed in the same manner as with theapparatus shown in FIG. 8.

Next, the stopper 8 is lifted and the molten metal 3-a in which the gashas been dissolved is poured through a molten metal inlet 11 into a meltholding container 19 located at the bottom of the molten metal holdingsection 22. Before the molten metal is poured into the melt holdingcontainer 19, a vacuum pump (not shown) is actuated to purge the gasthrough the degassing pipe 31 to thereby reduce the pressure inside themolten metal holding section 22, after which a given gas is introducedthrough a gas supply pipe 17 to maintain the inside at a given pressure.The gas pressure inside the molten metal holding section 22 can beeasily controlled by suitably opening or closing the gas supply pipe 17and a gas exhaust pipe 18. The molten metal that has been poured intothe melt holding container 19 is maintained at a given temperature by aheater 20.

Then, the molten metal that has been pressurized by the gas suppliedfrom a gas injection pipe 16 enters a mold 21 and is continuously cast,eventually forming a long porous metal body. The behavior of the gas atthe liquid phase/solid phase interface in the course of thesolidification of the molten metal, how the pores are formed in themetal body, and so forth are substantially the same as with theapparatus shown in FIG. 8. The main constituents of the continuouscasting apparatus include the portion of the mold 21 surrounded by acooling mechanism 25 (the liquid phase/solid phase interface is formedin this portion), an auxiliary cooling mechanism 26 which is providedoptionally, a guide spindle 27 which is contacted with the end of thesolidified porous metal body, rollers 28, and so forth. The continuouscasting apparatus is provided inside a sealed structure 30 in order toprevent the oxidation of the porous metal body at high temperatures, toprotect the cooling mechanism, and so on. The sealed structure 30 isequipped with an airtight ring 29, an inert gas injection pipe 23, andan inert gas exhaust pipe 24 in order to adjust the inert gas pressureinside this structure. In FIG. 11, at the point when the end of theporous metal body guided by the guide spindle 27 moving to the leftreaches the position where the airtight ring 29 is installed, theairtight ring 29 moves inward so as to come into close contact with theouter circumferential surface of the porous metal body. Then, the guidespindle 27 is taken out of the sealed structure 30, and the porous metalbody is then successively withdrawn out of the sealed structure 30.Thus, a long porous metal body is obtained.

FIG. 12 is a schematic diagram showing another example of the continuouscasting apparatus used for producing a long porous metal body. In FIG.12, the mechanical elements related to degassing and melting the rawmetal material are left out. With this apparatus, in the course ofsolidification, the liquid phase/solid phase interface of the metal isformed inclined to the movement direction of the metal body due to theeffect of the shape and the position of the cooling mechanism 26, thecooling rate, the gas pressure, and so forth, so that a porous metalbody having the inclined pores shown in the drawing is obtained. Theshape of the porous metal body can be any desired shape, such ascylindrical, linear, tabular, prismatic, etc., corresponding to theinternal surface shape of the mold.

FIG. 13 is a schematic diagram showing yet another example of thecontinuous casting apparatus used for producing a rod-shaped orwire-shaped porous metal body. Again in FIG. 13, the mechanical elementsrelated to degassing and melting the raw metal material are left out.With this apparatus as well, in the course of solidification, thestructure and the location of the cooling mechanism 26, the coolingrate, the gas pressure, and so forth are adjusted, and the liquidphase/solid phase interface in the metal is controlled with respect tothe movement direction of the metal body, producing a porous metal bodyhaving pores of the shape shown in the drawing.

FIG. 14 (a) to FIG. 14 (h) are schematic oblique views, with partialcut-aways, of the porous metal body manufactured by the method of thepresent invention by continuous casting process. For example, the porousmetal body shown in FIG. 14 (a) is a cylindrical metal body having across section corresponding to C₃ in FIG. 2, and can be manufacturedwhen the liquid phase/solid phase interface in the metal is moved at aconstant movement rate along the transverse cross section of thecylinder from one end to the other. The cylindrical porous metal bodyshown in FIG. 14 (b) is a cylindrical metal body having a cross sectioncorresponding to C₃ in FIG. 2, and can be manufactured when the movementrate of the liquid phase/solid phase interface in the metal is changedintermittently along the transverse cross section of the cylinder fromone end to the other. The cylindrical porous metal body shown in FIG. 14(c) is a cylindrical metal body having a cross section corresponding toC₃ in FIG. 2, and can be manufactured when the gas pressure is changedintermittently while the movement rate of the liquid phase/solid phaseinterface in the metal is constant along the transverse cross section ofthe cylinder from one end to the other. The cylindrical porous metalbody shown in FIG. 14 (d) is a cylindrical metal body having a crosssection corresponding to C₃ in FIG. 2, and can be manufactured when thegas pressure and the movement rate of the liquid phase/solid phaseinterface in the metal along the transverse cross section of thecylinder from one end to the other are changed intermittently. As shownin FIG. 10, the cylindrical porous metal body shown in FIG. 14 (e) canbe manufactured when the cooling mechanism 6 is located in the center ofthe mold and the liquid phase/solid phase interface in the metal ismoved in the transverse cross sectional direction from the center of thecylinder toward the peripheral portion. The cylindrical porous metalbody shown in FIG. 14 (f) can be manufactured when the cooling mechanismis located around the peripheral portion of the cylindrical mold and theliquid phase/solid phase interface in the metal is moved at a constantrate in the transverse cross sectional direction from the peripheralportion toward the center of the cylinder. In this case, a ring portionin which no pores are present can be formed around the periphery byperforming the initial cooling rapidly. The cylindrical porous metalbody shown in FIG. 14 (g) can be manufactured by the procedure shown inFIG. 1. The porous metal body shown in FIG. 14 (h), which has arectangular cross section, can be manufactured by the procedure shown inFIG. 11 with using a mold having a rectangular inner surface.

INDUSTRIAL APPLICABILITY

According to the present invention, it is possible to produce a porousmetal material with a pore shape and size, porosity, and so oncontrolled by an easy method using simple equipment.

According to the present invention, it is possible to manufacture aporous metal material of any shape desired.

When the present invention is implemented by a continuous castingmethod, large and long porous metal materials can be manufactured.

According to the present invention, it is possible to remarkably reducethe content of impurity components in the resulting porous metal body ascompared to the raw metal material. For instance, it is possible toreduce the oxygen content to 1/20 or less, and to reduce the nitrogencontent to ⅙ or less.

In the present invention, when iron or an iron alloy is used as the rawmetal material, and nitrogen is used as the pressurizing gas component,a nitriding phase is formed on all surfaces including the internalsurfaces of the pores, resulting in a marked increase in hardness.

The porous metal material obtained according to the present invention islightweight, has high specific strength (strength/weight), and hasexcellent machinability, weldability and so forth.

Also, the porous metal material according to the present invention canform a novel composite material that exhibits distinctive performance byfilling its pore portions with another material or supporting anothermaterial in its pore portion. As a specific example of such a compositematerial, a catalyst whose carrier is a porous metal body instead of aconventional honeycomb carrier (such as an exhaust gas treatmentcatalyst for automobiles and so on, a deodorizing catalyst, etc.) wouldbe exemplified.

In the present invention, the safety of the operations can be greatlyimproved if nitrogen, argon, helium or other such nonflammable gas isused as the pressurizing gas.

Because of its unique structure and excellent characteristics, theporous metal body according to the present invention can be utilized ina wide range of fields. Examples of such fields include hydrogen storagematerials, vibration-proof materials, shock absorbing materials,electromagnetic shielding materials, parts and structural materials invarious structures (engine parts for vehicles such as automobiles,ships, airplanes and so forth, ceramics supports for rocket and jetengines, lightweight panels for space equipment, machine tool parts,etc.), medical device materials (such as stent materials, etc.), heatexchange materials, sound insulation materials, gas/liquid separationmaterials, lightweight structural material parts, water and gaspurification filters, self-lubricating bearing materials, gas blowingmaterials in gas/liquid reactions, and so forth. The porous metal bodyaccording to the present invention is not limited to the aboveapplications, and can be utilized in various other applications as well.

BEST MODE FOR CARRYING OUT THE INVENTION

The best modes (examples) of the present invention will be given belowto further clarify the characteristics of the present invention. Thepresent invention is not limited to the following examples, and it goeswithout saying that various alterations, modifications, changes, etc.,can be made within the scope of the present invention.

EXAMPLE 1

A porous copper material was manufactured by using the apparatus shownin FIG. 8.

More specifically, the copper raw material (99.99% purity) wasmaintained for 0.1 hour at 1250° C. and 5×10⁻² Torr, and then melted for0.5 hour at 1250° C. under an atmosphere of one of the pressurizinggases which will be described in detail below. Then, under the samepressurization conditions, the molten copper having the gas as dissolvedtherein was poured into a cylindrical mold (100 mm tall, 30 mm insidediameter) and solidified from the bottom to the top by means of a watercooling mechanism provided at the bottom of the mold, yielding a porouscopper cylinder with the structure shown in FIG. 14 (c).

Pressurizing Atmosphere Gas (Gauge Pressure)

(a) 0.2 MPa H₂+0.6 MPa Ar

(b) 0.4 MPa H₂+0.4 MPa Ar

(c) 0.6 MPa H₂+0.2 MPa Ar

(d) 0.8 MPa H₂

FIG. 15 shows the porosity each of the four different porous coppercylinders (a) to (d) obtained. It is clear from the results shown inFIG. 15 that under a constant pressure pressurization condition, theporosity increases as the hydrogen partial pressure rises.

FIG. 16 (a) to FIG. 16 (d) are electronically processed images(corresponding to optical micrographs) showing a portion of thetransverse cross section each of the above-mentioned four differentporous copper cylinders (a) to (d). These show that the pore size can bevaried by adjusting the argon/hydrogen partial pressure ratio.

FIG. 17 is an electronically processed image (corresponding to anoptical micrograph) illustrating a portion of a vertical cross sectionof the porous copper cylinder (c) obtained above. It is clear thatelongated pores aligned vertically have been formed in a regularpattern.

The copper raw material contained about 157 ppm oxygen and 13 ppmnitrogen, whereas the oxygen and nitrogen contents in the copper porousbody had dropped to 7 ppm and 2 ppm, respectively.

EXAMPLE 2

A porous iron material was manufactured by using the apparatusschematically shown in FIG. 8.

More specifically, iron raw material (99.99% purity) was maintained for0.1 hour at 1800° C. and 5×10⁻² Torr, and then melted for 0.5 hour at1650° C. under an atmosphere of one of the pressurizing gases describedin detail below. Then, under the same pressurization conditions, themolten iron having the gas as dissolved therein was poured into acylindrical mold (100 mm tall, 30 mm inside diameter) and solidifiedfrom the bottom to the top by means of a water cooling mechanismprovided at the bottom of the mold, giving a porous iron cylinder withthe structure shown in FIG. 14 (a).

Pressurizing Atmosphere Gas (Gauge Pressure)

(a) 0.3 MPa N₂+1.2 MPa He

(b) 1.0 MPa N₂+1.0 MPa He

(c) 1.0 MPa N₂+0.5 MPa He

(d) 1.5 MPa N₂+0.5 MPa He

FIG. 18 shows the porosity each of the four different porous ironcylinders (a) to (d) obtained. It is clear from the result shown in FIG.18 that under the pressurization condition of a constant pressure,porosity can be controlled by adjusting the nitrogen and helium partialpressures.

FIG. 19 (a) to FIG. 19 (d) are electronically processed images(corresponding to optical micrographs) showing a portion of thetransverse cross section each of the above-mentioned four differentporous iron cylinders (a) to (d). These show that the pore size can bevaried by adjusting the argon/hydrogen partial pressure ratio.

The porous iron materials obtained were heated to about 1000° C., andthen plunged into water to conduct hardening, with the result that theVickers hardness thereof increased about 2.5- to 3-fold.

EXAMPLE 3

A porous nickel material was manufactured by using the apparatusschematically shown in FIG. 8.

More specifically, the nickel raw material (99.99% purity) wasmaintained for 0.1 hour at 1600° C. and 5×10⁻² Torr, and then melted for0.5 hour at 1600° C. under a pressurizing gas atmosphere (0.6 MPa N₂+0.2MPa Ar). Then, under the same pressurization conditions, the moltennickel having the gas as dissolved therein was poured into a cylindricalmold (100 mm tall, 30 mm inside diameter) and solidified from the bottomto the top by means of a water cooling mechanism provided at the bottomof the mold, giving a porous nickel cylinder with the structure shown inFIG. 14 (a).

FIG. 20 shows a portion of a transverse cross section of the porousnickel cylinder obtained as an electronically processed image(corresponding to an optical micrograph).

EXAMPLE 4

A porous copper column (100 mm tall, 30 mm inside diameter) was producedby using the apparatus schematically shown in FIG. 8 and the moldschematically shown in FIG. 10, after which this column was converted toobtain a porous cylinder.

More specifically, the copper raw material (99.99% purity) wasmaintained for 0.1 hour at 1250° C. and 5×10⁻² Torr, and then melted for0.5 hour at 1250° C. under a pressurizing gas atmosphere (0.3 MPa H₂+0.6MPa Ar). Then, under the same pressurization conditions, the moltencopper having the gas as dissolved therein was poured into a cylindricalmold and solidified from the bottom to the top, yielding a porouscolumn. This column was then processed with a wire cutter to obtain aporous copper cylinder with the shape shown in FIG. 21 and having anoutside diameter of 20 mm and a thickness of 1 mm.

FIG. 22 is an electronically processed image (corresponding to anoptical micrograph) showing a portion of a horizontal cross section ofthe porous copper cylinder obtained. It is clear from this image thatpores have been formed extending from the inner surface of the cylinderto the peripheral surface.

FIG. 23 is an electronically processed image (corresponding to anoptical micrograph) showing a portion of the outer surface of the porouscopper cylinder shown in FIG. 22. It is clear from this image thatnumerous pores have been formed from the inner surface of the cylinderall the way to the outer peripheral surface.

EXAMPLE 5

A porous copper column (100 mm tall, 30 mm inside diameter) wasmanufactured by using the apparatus schematically shown in FIG. 8 andthe mold schematically shown in FIG. 10, and then this column wasconverted to obtain a porous cylinder.

More specifically, the copper raw material (99.99% purity) wasmaintained for 0.1 hour at 1250° C. and 5×10⁻² Torr, and then melted for0.5 hour at 1250° C. under a pressurizing gas atmosphere (0.3 MPa H₂+0.2MPa Ar). Then, under the same pressurization conditions, the moltencopper having the gas as dissolved therein was poured into a cylindricalmold and cooled from the bottom so that it solidified toward thecylindrical mold direction, yielding a porous copper column. This columnwas then converted with a wire cutter to obtain a porous copper cylinderwith the shape shown in FIG. 24 and having an outside diameter of 22 mmand a thickness of 1 mm.

The porous copper cylinder obtained had a such a high porosity thatlight transmission was visible to the naked eye.

FIG. 25 is an electronically processed image (corresponding to anoptical micrograph) showing a portion of a transverse cross section ofthe porous copper cylinder shown in FIG. 24. It is clear from this imagethat pores have been formed extending from the inner surface of thecylinder to the peripheral surface.

FIG. 26 is an electronically processed image (corresponding to anoptical micrograph) showing a portion of the outer surface of the porouscopper cylinder shown in FIG. 24. It is clear from this image thatnumerous pores have been formed from the inner surface of the cylinderall the way to the outer peripheral surface.

EXAMPLE 6

A porous copper column (100 mm tall, 30 mm outside diameter) wasmanufactured by using the apparatus schematically shown in FIG. 8 andthe mold schematically shown in FIG. 9.

More specifically, the copper raw material (99.99% purity) wasmaintained for 0.1 hour at 1250° C. and 5×10⁻² Torr, and then melted for0.5 hour at 1250° C. under a pressurizing gas atmosphere (0.4 MPa H₂+0.4MPa Ar). Then, under the same pressurization conditions, the moltencopper having the gas as dissolved therein was poured into a cylindricalmold and solidified toward the top of the cylindrical mold from thecooling surface at the bottom, yielding a porous copper cylinder withthe shape shown in FIG. 14( c).

A disk-shaped test piece of 3 mm thickness was cut from this cylinderand placed on a white paper. Light was irradiated from above, andformation of the numerous pores of a uniform pore size was confirmed, asshown in FIG. 27.

1. A process for producing a porous metal body, comprising: (1)maintaining under reduced pressure in the range between 10⁻¹ and 10⁻⁶Torr a raw metal material within a temperature range which is 50 to 200°C. lower than the melting point of the metal in a sealed vessel tothereby degas the raw metal material; (2) melting the raw metal materialunder pressurization of between 0.1 and 10 MPa by introducing a gascontaining nitrogen gas and one or more gases selected from the groupconsisting of hydrogen, argon, and helium into the sealed vessel tothereby dissolve the gas in the molten metal; and (3) pouring the moltenmetal into a mold equipped with a cooling apparatus while controllingthe gas pressure above and the temperature of the molten metal, coolingand solidifying the molten metal in the mold inside the sealed vessel toform a porous metal body.
 2. The process for producing a porous metalbody according to claim 1, wherein the raw metal material is selectedfrom the group consisting of iron, copper, nickel, cobalt, magnesium,aluminum, titanium, chromium, tungsten, manganese, molybdenum, berylliumand alloys comprising one or more of these metals.
 3. The process forproducing a porous metal body according to claim 1, wherein the pressureapplied in step (2) is between 0.2 and 2.5 MPa.
 4. The process forproducing a porous metal body according to claim 1, wherein the coolingand solidification of the molten metal in step (3) is performed by acontinuous casting method.
 5. The process for producing a porous metalbody according to claim 1, wherein said gas is a nitrogen-argon mixture,a nitrogen-helium mixture or a nitrogen-argon-helium mixture.
 6. Amethod for producing a porous metal comprising: holding a metal under apressure of between 10¹ and 10⁶ Torr and at a temperature lower by 50 to200° C. than the melting point of the metal in a sealed vessel, therebydegassing the metal; melting the metal under a pressure of between 0.1and 10 MPa while introducing a gas mixture containing nitrogen gas andat least one gas selected from the group consisting of hydrogen, argon,and helium into the sealed vessel, thereby dissolving a part of the gasin the molten metal; and pouring the molten metal into a mold, andcooling and solidifying the molten metal in the mold to produce a porousmetal.
 7. The method according to claim 6, wherein said gas mixture is anitrogen-argon mixture, a nitrogen-helium mixture or anitrogen-argon-helium mixture.
 8. The method according to claim 6,wherein the gas mixture contains hydrogen.
 9. A process for producing aporous metal body, comprising: (1) maintaining under reduced pressure inthe range between 10⁻¹ and 10⁻⁶ Torr a raw metal material within atemperature range which is 50 to 200° C. lower than the melting point ofthe metal in a sealed vessel to thereby degas the raw metal material;(2) melting the raw metal material under pressurization of between 0.1and 10 MPa by introducing a gas containing a nitrogen-argon mixture, anitrogen-helium mixture or a nitrogen-argon-helium mixture into thesealed vessel to thereby dissolve the gas in the molten metal; and (3)pouring the molten metal into a mold equipped with a cooling apparatuswhile controlling the gas pressure above and the temperature of themolten metal, cooling and solidifying the molten metal in the moldinside the sealed vessel to form a porous metal body.
 10. The processfor producing a porous metal body according to claim 9, wherein the rawmetal material is selected from the group consisting of iron, copper,nickel, cobalt, magnesium, aluminum, titanium, chromium, tungsten,manganese, molybdenum, beryllium, and alloys comprising one or more ofthese metals.
 11. The process for producing a porous metal bodyaccording to claim 9, wherein the pressure applied in step (2) isbetween 0.2 and 2.5 MPa.
 12. The process for producing a porous metalbody according to claim 9, wherein the cooling and solidification of themolten metal in step (3) is performed by a continuous casting method.13. A method for producing a porous metal comprising: holding a metalunder a pressure of between 10⁻¹ and 10⁻⁶ Torr and at a temperaturelower by 50 to 200° C. than the melting point of the metal in a sealedvessel, thereby degassing the metal; melting the metal under a pressureof between 0.1 and 10 MPa while introducing a nitrogen-argon mixture, anitrogen-helium mixture or a nitrogen-argon-helium mixture into thesealed vessel, thereby dissolving a part of the gas in the molten metal;and pouring the molten metal into a mold, and cooling and solidifyingthe molten metal in the mold to produce a porous metal.
 14. A processfor producing a porous metal body, comprising: (1) maintaining underreduced pressure in the range between 10⁻¹ and 10⁻⁶ Torr a raw metalmaterial within a temperature range which is 50 to 200° C. lower thanthe melting point of the metal in a sealed vessel to thereby degas theraw metal material; (2) melting the raw metal material underpressurization of between 0.1 and 10 MPa by introducing a nitrogen gasinto the sealed vessel to thereby dissolve the gas in the molten metal;and (3) pouring the molten metal into a mold equipped with a coolingapparatus while controlling the gas pressure above and the temperatureof the molten metal, cooling and solidifying the molten metal in themold inside the sealed vessel to form a porous metal body.